This invention relates to a high-performance resin composition for the production of printed circuit boards.
Circuit boards are widely used in electronic industries, such as the production of radio, TV sets, TV games, personal computers, various kinds of recorders and some other electronic apparatus. The production of circuit boards is firstly to impregnate a woven fiberglass sheet with resin, and then laminate copper sheet to one or both sides of the resin treated fiberglass sheet via a pressing process, and finally etch electrical circuit into the copper layer. Electrical connections are usually soldered to the board when the board is used.
Polymide resins are usually adopted to impregnate the fiberglass sheets to form high-quality circuit boards which have good thermal resistance, low thermal expansion coefficient and good electrical properties including high resistivity. However, in comparison with the epoxy-made circuit boards, such polyimide boards are relatively too expensive. Nevertheless, the circuit boards made from epoxy resin impregnated fiberglass sheets are relatively poor in thermal resistance, dimensional stability and electrical properties.
Therefore, there is a need for a high-performance composition which is cost competitive to epoxy resin and has physical and electrical properties close to polyimide resin. Previously, many patents and reports were disclosed concerning the improvement of epoxy resin or polyimide resin for the production of printed circuit boards. Some important patents of this subject include U.S. Pat. Nos. 4,529,790, 4,559,395, 4,594,921, 2,947,726, 2,971,942, 2,809,942 and 4,294,877. An effective improvement for this purpose is to adopt a difunctional epoxy resin, however, the glass transition temperature (Tg) of it is not high enough to suit the high production temperature. In the production of printed circuit boards, generally, one important requirement is that the glass transition temperature should be higher than 180.degree. C., otherwise, distortion will occur during high-temperature operation.
To eliminate the drawback of conventional difunctional epoxy resin, a method as disclosed in U.S. Pat. No. 4,294,877 was to add bismaleimide (BMI) in the epoxy resin to increase the glass transition temperature. However, though the addition of bismaleimide would improve the heat resistance of epoxy resin, it would also cause the resultant printed circuit boards to become very brittle. The brittleness of the bismaleimide modified epoxy resin composition would result in much scrap when the cured product is drilled. In order to improve the brittleness, some suitable rubbers have been reported to be added in the resin system. However, the addition of rubber would unavoidably result in phase separation which would destroy the physical properties of circuit boards.